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trees move up but the treeline doesn’t

Work from our team has showed provided evidence that cloudforest tree species from the Andes and from Costa Rica are shifting their distribution upslope, possibly in response to increasing temperatures. For the Andes, we conducted a follow-up study where we predicted the future population sizes, and thus extinction vulnerabilities, of the migrating species under different warming scenarios and sets of competing assumptions. What this exercise clearly showed is that the future of these cloudforest species depends very much on what happens at the upper limit of their distributions – the treeline*. If species are able to migrate upslope and extend their ranges past the current treeline to occupy the parts of the puna (high elevation grassland habitat above treeline sometime referred to as paramo in other parts of the Andes) that become climatically suitable, then we predict that their population sizes may actually increase. The reason for the increase, and apparent benefit of climate change, is simply that the cross section of the Andes is more trapezoidal than triangular; consequently, as species migrate up off the steep slopes, past treeline and onto the high plateau (altiplano), the amount of land area that they can occupy increases. In contrast to this relatively rosy scenario, if the treeline remains where it currently is and doesn’t shift upslope with warming (e.g., due to cattle grazing and human activities above the treeline), we predict that all cloudforest species will suffer massive decreases in available habitat area and their population sizes (and hence increases in their risks of extinction) as the lower elevations become ‘too hot’ but they are unable to occupy the ‘just right’ temperatures at higher elevations.

So we need to know, “will treeline move with warming?” Well, we now have several decades of climate change behind us so we can look back and ask if treeline has moved or not in response to the 0.5-1oC of warming that the Andes have already experienced over the last several decades. As part of his dissertation research, our colleague, Przemek Zelazowski, looked at Landsat images collected over the Andes from 1970 to 2000. Due to warming, treeline should have shifted upslope between 90 and 140 vertical meters over this time period. He found zero change. Yes, treeline moved up in some places, but in other places it moved down and the modal change, by far, was zero zero zero. Now, a new study by David Lutz et al. conducts a similar analysis but using much higher-resolution imagery of the areas around Manu National Park where my team works. Over the 4 decades for which they had imagery, they found that 80% of the treelines that they looked at showed zero net change, and the average change across all treelines they looked at was an upslope shift of 0.14 vertical meters per year. This is in the right direction, but it is just 1% the rate that was required to keep pace with concurrent warming.

Treeline has not moved in response to past warming. It is doubtful that it will move in response to future warming. Cloudforest species will continue to shift their disruptions upslope. They will be unable to invade the puna. They will suffer decreases in their population sizes. They will be at danger of extinction.

All bad news, right? Well there is one result from the Lutz et al. study that may be the silver lining on an otherwise very dark cloud. When Lutz et al. compared the rates of movement of the treelines in the protected areas of Manu National Park to the rates of movement of the treelines in unprotected areas outside the park, they found that protected treelines migrated upslope 5 times faster than their unprotected counterparts (0.24 vs. 0.05 vertical meters per year). The rate of migration in the park is still way too slow, but this difference does indicate that parks and protection work. If we could make the parks better park (e.g, remove cattle and stop fires) and make the parks bigger, then at least some Andean treelines might move faster and some extinctions may be avoided.

-Ken Feeley

*My grad student, Evan Rehm, is probably cringing at my use of the word ‘treeline’. I will leave it up to him to define the correct use of ‘treeline’ vs. ‘timberline’ vs. ‘tree species line’ and to better explain why the treeline or timberline is not shifting upslope faster in Manu.